I replaced my old AC only thermostat with Honeywell RTH6580WF smart Thermostat. Also replaced existing 3 terminal wire with 4 for C wire hookup.

TSTAT specs h t t p yourhome.honeywell.com/en/products/thermostat/wi-fi-7-day-programmable-thermostat-rth6580wf

Current TSTAT wiring.

RED - R (jumper R and Rc) YELLOW - Y (color has faded on it as you can see in image, I think its brown color, which is connected to white terminal from green pig tailed brown wire coming from outside fan unit) GREEN - G

AC only wiring

Below is old picture of TSTAT wire (brown wire non pig tailed) connected to furnace wiring before I snaked 4 terminal wire.

TSTAT and Outside FAN connection

On furnace side there are 2 brown wires, 1 has green wire pig tailed and other don't. The one which is pig tailed is coming from outside FAN unit while other brown wire is coming from TSTAT. Any idea where should I connect the blue wire from TSTAT to furnace side C Wire? (you don't see in attached picture as its the old TSTAT wire with 3 terminal)

I am thinking it should be connected to Brown and Red wire (FIG-2) using wire nut. I do get 28v between original Red and Brown wire making me think that brown is C wire. Is that right?


  • Was the old thermostat powered or mechanical? (like one of the round ones) Just because there is 24V between the two wires does not mean that you have a common. If you are able to open up your AC and see if one of them runs to the transformer, then it's possible. Otherwise you will have to run a new wire because you need a common to power the new thermostat. – TFK May 3 '17 at 19:35
  • Old TSTAT has 2 AAA batteries in it. – nil May 3 '17 at 19:43

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